Method for heat treating synthetic quartz glass for optical use, heat treatment apparatus for the same, and synthetic quartz glass for optical use

ABSTRACT

An object of the present invention is to overcome the problems of the prior art technique, and to provide a heat treatment method as well as a heat treatment apparatus capable of heat treating, with higher efficiency, a synthetic quartz glass for optical use having higher homogeneity and higher purity. Another object of the present invention is to provide and a synthetic quartz glass for optical use.  
     The problems above are solved by, in a method for heat treating a flat cylindrical synthetic quartz glass body provided as the object to be heat treated in a heating furnace, a method for heat treating a synthetic quartz glass for optical use comprising preparing a vessel made of quartz glass and having a flat cylindrical space for setting therein the object synthetic quartz glass body, placing two or more object synthetic quartz glass bodies into the vessel in parallel with each other, filling the space with SiO 2  powder, setting the vessel inside the heating furnace with its lid closed, and applying the heat treatment to the vessel.

TECHNICAL FIELD TO WHICH THE INVENTION BELONGS

[0001] The present invention relates to a highly homogeneous syntheticquartz glass having high optical transmittance and high resistanceagainst laser radiations, which is suitable for use as an optical memberto be employed in a lithographic apparatus equipped with an excimerlaser. It also relates to a method for heat treating and to a heattreatment apparatus for said synthetic quartz glass.

PRIOR ART

[0002] The technology of photolithography, which comprises transferringa pattern provided on a photomask to a wafer by using a laser radiation,is widely used for aligners for producing semiconductor integratedcircuits, because of its superiority on production cost over othertechniques utilizing electron beams and X rays.

[0003] Recently, with an increase in miniaturization and in the degreeof integration of LSIs, light sources with shorter wavelengths are usedfor the exposure, and aligners employing an i-line (having a wavelengthof 365 nm) capable of forming patterns with a line width of from 0.4 to0.5 μm or a KrF excimer laser radiation (having a wavelength of 248.3nm) capable of forming patterns with a line width of from 0.25 to 0.35μm have been used in practice. More recently, the development of analigner equipped with an ArF excimer laser (having a wavelength of 193.4nm) capable of forming patterns with a line width of from 0.13 to 0.2 μmis progressing for its practical use. Thus, in an optical member for usein the lithographic apparatus equipped with an ArF excimer laser, it isrequired that it more satisfies the requirements such as homogeneity,transmittance, resistance against laser radiations, etc., at an everhigher level.

[0004] As a material capable of satisfying the above requirements for anoptical member, a high purity synthetic quartz glass has been utilized,and improvements in the optical transmittance and the resistance againstlaser radiations have been made on those materials by optimizing theconditions of production. At the same time, further improvement inoptical characteristics such as homogeneity and birefringence is beingattempted. Among them, in order to achieve an improvement in homogeneityand a reduction of birefringence, it is known that a heat treatmentaccompanying gradual cooling (annealing treatment) for the strainremoval of quartz glass should be incorporated during the productionprocess of an optical member. As such a heat treatment, generally knownis to keep the optical member at a high temperature inside the heatingfurnace for a long duration of time.

[0005] However, on lowering the temperature in the annealing treatmentabove, a difference in temperature distribution was found to generatebetween the central portion and the outer peripheral portion of theobject to be treated, and this temperature distribution was found toremain as a difference in density even after the annealing treatment.Hence, the improvements on the distribution of refractive indices and onbirefringence were found still insufficient.

[0006] Accordingly, in order to further improve the distribution ofrefractive indices and the birefringence of quartz glass, there had beenproposed a method comprising performing the annealing by placing theobject to be treated inside a ring or a vessel, or buried in a powder,thereby controlling the temperature distribution by retarding the rateof lowering the temperature on the outer peripheral side of the object.In case of employing such methods, preferred is to use a vessel having ashape as such that is thicker for the outer peripheral direction, so asto suppress the temperature distribution in the object, particularly,the temperature difference between the outer peripheral portion and thecentral portion, from occurring during lowering the temperature.

[0007] However, in case this method is employed, the number of theobjects to be set inside the heating furnace becomes extremely limitedas compared to the case not using a vessel; hence, this led to a poorproduction efficiency due to the decrease in the number of objectstreated at a single heat treatment.

PROBLEMS THE INVENTION IS TO SOLVE

[0008] The present invention has been made in the light of theaforementioned problems, and an object of the present invention is toprovide a heat treatment method and a heat treatment apparatus capableof heat treating, with higher efficiency, a synthetic quartz glass foroptical use having higher homogeneity and higher purity. It also is anobject of the present invention to provide a synthetic quartz glass foroptical use.

MEANS FOR SOLVING THE PROBLEMS

[0009] The objects of the present invention described above can beachieved by the constitutions (1) to (11) below according to the presentinvention.

[0010] (1) In a method for heat treating a flat cylindrical syntheticquartz glass body provided as the object to be heat treated in a heatingfurnace, a method for heat treating a synthetic quartz glass for opticaluse comprising preparing a vessel made of quartz glass and having a flatcylindrical space for setting therein the object synthetic quartz glassbody, placing two or more object synthetic quartz glass bodies into thevessel in parallel with each other, filling the space with SiO₂ powder,setting the vessel inside the heating furnace with its lid closed, andapplying the heat treatment to the vessel.

[0011] (2) A method for heat treating a synthetic quartz glass foroptical use as described above in constitution (1), wherein, in case twoor more of the object synthetic quartz glass bodies have the samediameter, the diameter of the space for placing the objects is provided2.1 times or more of the diameter of the synthetic quartz glass body.

[0012] (3) A method for heat treating a synthetic quartz glass foroptical use as described above in constitution (1) or (2), wherein theplace for placing the object synthetic quartz glass bodies inside thevessel is provided within 80% of the outer diameter of the vessel.

[0013] (4) A method for heat treating a synthetic quartz glass foroptical use as described in one of constitution (1) to (3) above,wherein said SiO₂ powder is a synthetic SiO₂ powder containing 30 ppb orless of Na.

[0014] (5) A method for heat treating a synthetic quartz glass foroptical use as described in one of constitution (1) to (4) above,wherein said SiO₂ powder is a SiO₂ powder having a hydrogen moleculedensity of 1.0×10¹⁹ molecules/cm² or higher.

[0015] (6) A method for heat treating a synthetic quartz glass foroptical use as described in one of constitution (1) to (5) above,wherein said SiO₂ powder contains 95% by weight or more of particleshaving a particle diameter of 1,000 gm or smaller.

[0016] (7) A synthetic quartz glass for optical use prepared by methodsas described in one of constitution (1) to (6) above, having afluctuation in refractive index An of 1.0×10⁻⁶ or less, a birefringenceof 0.5 nm/cm or lower, and an initial transmittance for a light 193.4 nmin wavelength of 99.7% or higher.

[0017] (8) In an apparatus for heat treating a flat cylindricalsynthetic quartz glass body provided as the object to be heat treated ina heating furnace, an apparatus for heat treating a synthetic quartzglass for optical use comprising a vessel made of quartz glass andprovided with a flat cylindrical space for setting therein the objectsynthetic quartz glass body, and SiO₂ powder filled so as to cover twoor more object synthetic quartz glass bodies set inside the vessel inparallel with each other.

[0018] (9) An apparatus for heat treating a synthetic quartz glass foroptical use as described in constitution (8) above, wherein, in case twoor more of the object synthetic quartz glass bodies have the samediameter, the diameter of the space for placing the objects is provided2.1 times or more of the diameter of the synthetic quartz glass body.

[0019] (10) An apparatus for heat treating a synthetic quartz glass foroptical use as described in constitution (8) or (9) above, wherein, saidSiO₂ powder is a synthetic SiO₂ powder containing 30 ppb or less of Na.

[0020] (11) An apparatus for heat treating a synthetic quartz glass foroptical use as described in one of constitution (8) to (10) above,wherein said SiO₂ powder contains 95% by weight or more of particleshaving a particle diameter of 1,000 μm or smaller.

MODE OF CARRYING OUT THE INVENTION

[0021] The heat treatment apparatus for synthetic quartz glass foroptical use according to the present invention is equipped with aheating furnace for heat treating the object to be treated, i.e., asynthetic quartz glass body. Since a heating furnace conventionally usedin the art can be used as it is, further explanation therefor isomitted. Furthermore, the heat treatment apparatus for synthetic quartzglass for optical use according to the present invention is equippedwith a lidded vessel made of a quartz glass, which is placed inside theheat furnace with two or more of the synthetic quartz glass bodies beingenclosed therein at the heat treatment.

[0022] As shown in FIG. 1, the lidded vessel 10 is provided with a flatcylindrical space 10 a having a diameter of at least two times of thediameter of the flat cylindrical synthetic quartz glass body G (in casethe two or more synthetic quartz glass bodies G have the same diameter),which is the object enclosed therein to be subjected to the treatment,and has a surrounding side wall 12, a disk-shaped bottom plate 14, and adisk-shaped lid 16. The surrounding side wall 12 and the bottom plate 14may be provided either separately or integrated monolithically.

[0023] The diameter of the space 10 a above is preferably 2.1 times ormore of the diameter of the synthetic quartz glass body G that is theobject to be treated therein. The volume of the lidded vessel 10 ispreferably 1.8 times or more, and particularly preferably, about 2 to 10times as large as that of the synthetic quartz glass that is the objectto be treated therein.

[0024] Since the synthetic quartz glass body G, i.e., the object to betreated, is 70 to 400 mm in diameter and about 5 to 150 mm in thickness,preferably, the space of the lidded vessel 10 is practically about 150to 1500 mm in diameter, about 10 to 200 mm in height, and about 200 to300,000 cm³ in volume. Furthermore, the space of the vessel 10preferably has a diameter to height ratio of 1.8 or more, andparticularly preferably, 2 or more. Although there is no upper limit forthe ratio, it is believed to be about 10.

[0025] As shown in FIG. 1, two or more of synthetic quartz glass bodiesG, which are the object to be treated, are placed inside the space 10 aof the vessel 10 in parallel with each other (that is, they are placedon the same plane), and, in this case, the position (denoted as a sampleposition in the Examples below) of the synthetic quartz glass bodies Gthat are the object to be treated is preferably within 80% of the outerdiameter of the vessel 10.

[0026] Furthermore, the vessel 10 above preferably contains Na at aconcentration of 100 ppb or less, particularly preferably 40 ppb orless, and further preferably, 5 ppb or less. Although the amount of Nathat reaches the synthetic quartz glass body G is limited because Nadischarged from the vessel, etc., is mostly trapped by the SiO₂ powderdescribed hereinafter, the Na content of the vessel 10 above ispreferably limited in the range above.

[0027] Further, the heat treatment apparatus according to the presentinvention is equipped with SiO₂ powder 20 (see FIG. 1) that is filled inthe vessel 10 above in such a manner that it covers the surroundings ofthe synthetic quartz glass body G that is the object to be treated. Asthe SiO₂ powder, there can be used a SiO₂ powder previously subjected toa high pressure hydrogen treatment and thereby doped with hydrogen. Inthis case, the concentration of hydrogen molecules that are incorporatedin the object to he treated can be maintained higher. The density of thehydrogen molecules dissolved in the hydrogen-doped SiO₂ powder ispreferably 1×10¹⁹ molecules/cm³ or higher in average. The density of thedissolved hydrogen molecules may be the average taken over the entireSiO₂ powder, and hence, the powder thus doped with hydrogen may be usedmixed with a powder not doped with hydrogen. The Na content of the SiO₂powder above is preferably 30 ppb or lower, and particularly preferably,10 ppb or lower. The lower the content of Na, the better. The SiO₂powder above consists of particles in which 95% by weight or morethereof have a particle diameter in a range of 1,000 μm or smaller,preferably, in a range of from about 0.1 to 1,000 μm, and morepreferably, in a range of from about 0.5 to 500 μm. If the range of theparticles should have a particle size too large, there is fear that thepowder fails to achieve the sufficiently high heat retentability due toa decrease in the packing density; hence, it is preferred that a powdercontaining particles exceeding 1,000 μm in particle diameter is notused. If the particle diameter should be too small, there occursproblems in handling; hence, it is practically desired not to use apowder having a particle diameter smaller than 0.1 μm. Further, in caseof using a powder doped with hydrogen, it is preferred not use a powdercontaining particles having a particle diameter exceeding 1,000 μm inparticle diameter because a particle too large in diameter there is fearof causing difficulty in dissolving sufficiently high amount of hydrogenmolecules to a part of the powder in case of applying high pressurehydrogen treatment and the like. Also from this point of view, it ispreferred that a powder having a particle diameter exceeding 1,000 μm inparticle diameter is not used. Concerning the packing density above, apowder mixture consisting of powders differed in particle diameter maybe used so long as the particle diameter fall within a range of 0.1 to1,000 μm, and it is more preferred to use a powder having a wider rangein particle diameter distribution, because such a powder often increasesthe packing properties. However, the effect of the present invention canbe achieved so long as the particle diameter of the SiO₂ powder usedsubstantially fall within the range above. More specifically, there isno practical problem even if less than 5% of the entire weight of theSiO₂ powder above should fall outside of the particle size range definedabove.

[0028] Further, by taking various conditions into consideration, it isparticularly preferred that the SiO₂ powder above is a synthetic quartzglass powder. Since the synthetic quartz glass vessel and the SiO₂powder above can be reused, it is economically advantageous to properlyuse them repeatedly depending on the size and the number of treatment.

[0029] The method for heat treating a synthetic quartz glass for opticaluse by performing a heat treatment in the heating furnace according tothe present invention comprises using the apparatus described above,and, as shown in FIG. 1, performing the heat treatment by placing aplurality of the objects of treatment, i.e., the synthetic quartz glassbodies G, in parallel with each other inside the vessel 10, covering thesurroundings of the synthetic quartz glass bodies G with SiO₂ powder 20,placing the vessel 10 with its lid 16 closed in the heating furnace, andapplying the heat treatment in this state. The treatment may beperformed in air. The temperature and the time of retention, the heatingrate, the cooling rate, and other thermal treatment conditions may bethose generally used in an ordinary heat treatment. In accordance withthe heat treatment method according to the present invention asdescribed above during lowering the temperature, the temperaturedistribution within the objects of the treatment, i.e., the syntheticquartz glass bodies G, the SiO₂ powder 20, and the vessel 10 as a wholeresults as such shown in FIG. 2. Hence, the temperature gradient asviewed along the direction of the radius of the synthetic quartz glassbody alone, i.e., the very object of the treatment, can be minimized.Accordingly, the fluctuation in the refractive index and thebirefringence thereof as viewed along the direction of the radius of thesynthetic quartz glass body can be minimized. Hence, in accordance withthe present invention, there can be obtained a synthetic quartz glassfor optical use having a fluctuation in refractive index An of 1.0×10⁶or less and a birefringence of 0.5 nm/cm or lower.

[0030] Further, the synthetic quartz glass for optical use according tothe present invention preferably contains Na at a concentration of 10ppb or less, and particularly preferably, 5 ppb or less. By reducing theNa content, the initial transmittance for a light 193.4 nm in wavelengthcan be improved, and it furthermore enables a synthetic quartz glass foroptical use having an initial transmittance of 99.7% or higher.

[0031] Still further, the dissolved hydrogen molecule density of thesynthetic quartz glass for optical use according to the presentinvention is preferably 2×10¹⁷ molecules/cm³ or higher, and morepreferably, 5×10¹⁷ molecules/cm³ or higher.

EXAMPLES

[0032] An embodiment for practicing the present invention is describedmore specifically below by partially making reference to the drawings.However, it should be understood that the size, materials, shapes,relative arrangement, etc., that are described in the embodiment beloware provided simply as examples or explanatory means, and are by nomeans limiting the present invention.

[0033] As a sample of a synthetic quartz glass for use as an opticalmember (sometimes referred to simply hereinafter as “sample”), which isthe object of the treatment, three synthetic quartz glass bodies each300 mm in diameter and 80 mm in thickness prepared by direct method wereeach used in Examples 1 and 2 and in Comparative Examples 1 and 2. ForExample 3, two synthetic quartz glass bodies each 340 mm in diameter and80 mm in thickness prepared similarly by direct method were used. Thesynthetic quartz glass bodies thus prepared each yielded a Naconcentration of 5 ppb or lower and a hydrogen molecule (H₂) density of1.8×10¹³ molecules/cm³. The synthetic quartz glass bodies above wereeach subjected to a heat treatment in air under a temperature profileshown in FIG. 3 in accordance with a method described below.

Example 1

[0034] A synthetic quartz glass vessel (which is sometimes simplyreferred to hereinafter as “vessel”) having an outer diameter of 900 mm,an inner diameter of 800 mm, a height of 200 mm, a space with height of100 mm, and a wall thickness of 50 mm, and a high purity syntheticquartz glass powder (which is sometimes simply referred to hereinafteras “powder”) substantially free of hydrogen molecules and having a Naimpurity concentration of 20 ppb, which consists of particles 53 to 710μm in particle diameter, were prepared.

[0035] Then, the sample described hereinbefore was placed inside theabove vessel in a state as shown in FIG. 1 while filling the powderdescribed above, and they were set inside a heating furnace in thisstate to perform the heat treatment. The samples were placed inside thevessel by taking an interval of 50 mm (as measured between the sideplanes) at a position accounting for 78% of the vessel.

Example 2

[0036] A heat treatment was performed in the same manner as thatdescribed in Example 1, except for taking an interval of 120 mm betweenthe samples and setting the samples at a position accounting for 87% ofthe vessel.

Example 3

[0037] A heat treatment was performed in the same manner as thatdescribed in Example 1, except for using, instead of the high puritysynthetic quartz glass powder, a powder of a naturally occurring quartz(rock crystal), i.e., IOTA (trademark) powder consisting of particles 53to 710 μm in particle diameter subjected to a purification treatmentunder a chlorine-containing atmosphere and thereby having a Naconcentration of 50 ppb, and for using two samples each having adiameter of 340 mm set at a distance of 20 mm from each other.

Comparative Example 1

[0038] A heat treatment was performed in the same manner as thatdescribed in Example 1, except for placing the sample inside the samelidded vessel as that used in Example 1 but without using a syntheticquartz glass powder.

Comparative Example 2

[0039] A heat treatment was performed in the same manner as thatdescribed in Example 1, except for placing the sample as it is insidethe heating furnace without using any powder or vessel.

[0040] The interval between the samples, the sample position, and the Nacontent of the SiO₂ powder used for filling are summarized in the tablegiven in FIG. 4.

[0041] The fluctuation in refractive index An before and after the heattreatment, birefringence after the treatment, the transmittance inpercent for a radiation 193.4 nm in wavelength after the treatment, andthe Na concentration after the treatment were measured on each of thesynthetic quartz glass samples obtained as the object of the treatmentin Examples and Comparative Examples above. The results are showncollectively in the table given in FIG. 4.

[0042] The table in FIG. 4 clearly reads that, for the three samples(synthetic quartz glass for optical use) treated in Example 1, thefluctuation in refractive index An decreases to 0.7 or lower after theheat treatment, that the birefringence is as low as 0.30 nm/cm, that theinitial transmittance for a radiation 193.4 nm in wavelength is 99.8%,and that the Na concentration is 2 ppb. Thus, all of the samplesexhibited sufficiently high homogeneity and ultraviolet transmittingproperties for use as an optical member to be equipped in a lithographicapparatus.

[0043] In the synthetic quartz glass for optical use treated inaccordance with Example 2, it reads that the fluctuation in refractiveindex An before and after the treatment is 1.0 or lower, that thebirefringence is 0.42 nm/cm or lower, that the transmittance for aradiation 193.4 nm in wavelength is 99.8%, and that the Na concentrationis 2 ppb. Thus, all of the samples exhibited sufficiently highhomogeneity and ultraviolet transmitting properties for use as anoptical member to be equipped in a lithographic apparatus, although theywere slightly inferior to those obtained in Example 1 in terms ofhomogeneity.

[0044] In the synthetic quartz glass for optical use treated inaccordance with Example 3, it reads that the fluctuation in refractiveindex An before and after the treatment is 0.9 or lower, that thebirefringence is 0.45 nm/cm or lower, and that the transmittance for aradiation 193.4 nm in wavelength is 99.8% or 99.7%. Thus, all of thesamples exhibited sufficiently high homogeneity and ultraviolettransmitting properties for use as an optical member to be equipped in alithographic apparatus, although they were slightly inferior to thoseobtained in Example 1.

[0045] In contrast to above, in the treated samples of ComparativeExample 1, the fluctuation in refractive index An before and after theheat treatment is found to be in a range of 1.2 to 1.8, thebirefringence is in a range of 0.61 to 0.99 nm/cm, and the initialtransmittance for a radiation 193.4 nm in wavelength for all the samplesis found to be 99.7%. Thus, although slightly inferior to those obtainedin Example 1, the samples exhibited sufficient ultraviolet transmittingproperties for use as an optical member to be equipped in a lithographicapparatus, but problems were found thereon concerning homogeneity.

[0046] In the treated samples of Comparative Example 2, the fluctuationin refractive index An before and after the treatment is found to be ina range of 2.2 to 2.7, the birefringence is in a range of 1.32 to 1.84nm/cm, and the initial transmittance for a radiation 193.4 nm inwavelength is found in a range of 99.5 to 99.6%. Thus, they had problemsconcerning both homogeneity and ultraviolet transmitting properties foruse as an optical member to be equipped in a lithographic apparatus.

[0047] From the results above, the effect of the present invention canbe clearly understood.

[0048] The methods for measuring the physical properties and the like asdescribed in Examples and Comparative Examples are as follows.

[0049] 1) Method for measuring the fluctuation in refractive index An: Ameasuring method according to optical interference method using a He—Nelaser (emitting radiation at a wavelength of 633 nm) as a light sourcewas used. In the measurement above, the values are given for an areaaccounting for 90% of the sample diameter.

[0050] 2) Method for measuring birefringence: A retardation measuringmethod using a polarizer strain meter was used. In the measurementabove, the values are given for an area accounting for 90% of the samplediameter.

[0051] 3) Method for measuring the transmittance for a radiation 193.4nm in wavelength:

[0052] A measurement method comprising obtaining an apparenttransmittance T % for a sample thickness of 10 mm, and calculating thevalue in accordance with (T/90.68)×100, by using the value 90.68%obtained by subtracting the loss due to Rayleigh scattering 0.18% fromthe theoretical transmittance 90.86% of a quartz glass for a radiation193.4 nm in wavelength.

[0053] 4) Method for measuring Na impurity concentration: A method usingflameless atomic absorption spectroscopy was used. 5) Method formeasuring the particle diameter of the synthetic quartz glass powder:The powder was classified by using JIS standardized sieves having Nylonscreens with apertures of 53 μm and 710 μm.

[0054]FIG. 1 is a diagram showing the main portion, i.e., the vessel andthe SiO₂ powder given together with the object synthetic quartz glassbody, of an apparatus for heat treating a synthetic quartz glass foroptical use according to the present invention, wherein FIG. 1(A) showsthe planar section view, and FIG. 1(B) shows the cross section viewtaken along line I-I of FIG. 1(A).

[0055]FIG. 2 is a diagram showing the temperature distribution in theobject synthetic quartz glass body during cooling inside an apparatusfor heat treating a synthetic quartz glass for optical use according tothe present invention, wherein FIG. 2(A) shows the planar section view,and FIG. 2(B) shows the cross section view taken along line II-II ofFIG. 2(A).

[0056]FIG. 3 is a diagram showing an example of a temperature profileused in a method for heat treating a synthetic quartz glass for opticaluse according to the present invention.

[0057]FIG. 4 is a table showing the physical properties and the like ofthe object before and after the heat treatment in accordance with theExamples and Comparative Examples of the present invention.

[0058] [Explanation of the Symbols]

[0059]10: Vessel

[0060]12: Surrounding side wall

[0061]14: Bottom plate

[0062]16: Lid

[0063]20: SiO₂ powder

[0064] G: Object synthetic quartz glass body

1. A method for heat treating a flat cylindrical synthetic quartz glassbody for optical use provided as the object to be heat treated in aheating furnace, comprising preparing a vessel made of quartz glass andhaving a flat cylindrical space for setting therein the object syntheticquartz glass body, placing two or more object synthetic quartz glassbodies into the vessel in parallel with each other, filling the spacewith SiO₂ powder, setting the vessel inside the heating furnace with itslid closed, and applying the heat treatment to the vessel.
 2. A methodfor heat treating a synthetic quartz glass for optical use as claimed inclaim 1, wherein, in case two or more of the object synthetic quartzglass bodies have the same diameter, the diameter of the space forplacing the objects is provided 2.1 times or more of the diameter of thesynthetic quartz glass body.
 3. A method for heat treating a syntheticquartz glass for optical use as claimed in claim 1 or 2, wherein theplace for placing the object synthetic quartz glass bodies inside thevessel is provided within 80% of the outer diameter of the vessel.
 4. Amethod for heat treating a synthetic quartz glass for optical use asclaimed in one of claims 1 to 3, wherein said SiO₂ powder is a syntheticSiO₂ powder containing 30 ppb or less of Na.
 5. A method for heattreating a synthetic quartz glass for optical use as claimed in one ofclaims 1 to 4, wherein said SiO₂ powder is a SiO₂ powder having ahydrogen molecule density of 1.0×10¹⁹ molecules/cm² or higher.
 6. Amethod for heat treating a synthetic quartz glass for optical use asclaimed in one of claims 1 to 5, wherein said SiO₂ powder contains 95%by weight or more of particles having a particle diameter of 1,000 μm orsmaller.
 7. A synthetic quartz glass for optical use prepared by methodsas described in one of claims 1 to 6, having a fluctuation in refractiveindex An of 1.0×10⁶ or less, a birefringence of 0.5 nm/cm or lower, andan initial transmittance for a light 193.4 nm in wavelength of 99.7% orhigher.
 8. In an apparatus for heat treating a flat cylindricalsynthetic quartz glass body provided as the object to be heat treated ina heating furnace, an apparatus for heat treating a synthetic quartzglass for optical use comprising a vessel made of quartz glass andprovided with a flat cylindrical space for setting therein the objectsynthetic quartz glass body, and SiO₂ powder filled so as to cover twoor more object synthetic quartz glass bodies set inside the vessel inparallel with each other.
 9. An apparatus for producing a syntheticquartz glass for optical use as claimed in claim 8, wherein, in case twoor more of the object synthetic quartz glass bodies have the samediameter, the diameter of the space for placing the objects is provided2.1 times or more of the diameter of the synthetic quartz glass body.10. An apparatus for heat treating a synthetic quartz glass for opticaluse as claimed in claim 8 or 9, wherein, said SiO₂ powder is a syntheticSiO₂ powder containing 30 ppb or less of Na.
 11. An apparatus for heattreating a synthetic quartz glass for optical use as claimed in claim 8or 9, wherein said SiO₂ powder contains 95% by weight or more ofparticles having a particle diameter of 1,000 μm or smaller.